Coking refers to the decomposition of a hydrocarbon species into a solid mass consisting largely of carbon. This process can be promoted under conditions where a hydrocarbon is heated to elevated temperatures in the absence of hydrogen or an oxidant such as carbon dioxide, steam, or oxygen. The decomposition can be detrimental for various materials, such as catalysts, and can occur homogeneously or can be promoted over a heterogeneous catalytic surface.
Catalysts are materials used to promote a chemical reaction without being consumed. Commonly, a solid catalyst is used in the transformation of a liquid or gases species. Coking of hydrocarbons over a heterogeneous, solid catalyst often leads to a deactivation of the catalyst activity. Further, coke formation may cause mechanical damage to the catalyst particle leading to substantial pressure drop through the reaction vessel. Once the mechanical integrity of a catalyst is damaged, the catalyst typically needs to be replaced even if the catalyst activity could be recovered through a regeneration event.
Coking can occur over both oxide-based as well as metal-containing catalysts. Examples of reactions wherein coke formation over metal surfaces can be problematic include, for example, dry reforming and steam reforming. The steam reforming of methane and carbon dioxide or dry reforming of methane can be illustrated with the following equations:CH4+H2OCO+3H2  (1)CH4+CO22CO+2H2  (2)
In general, the carbon that may be formed can either be amorphous or crystalline/graphitic in nature. Crystalline coke may be produced from carbon which has been incorporated and then expelled from the metal. Although this is useful for the manufacture of carbon nanotubes, this dissolution and crystallization process can be damaging to the mechanical integrity of the catalyst. Graphitic coke is also more challenging to gasify than amorphous species. Similar coking problems often arise in the metal reactor tubes and heat exchangers used to process the reaction chemistry. Coking on these surfaces lowers the heat transfer into the reaction medium and can lead to both metal embrittlement as well as metal loss through dusting.
Beyond chemical transformations over heterogeneous catalysts and respective process equipment, coking may also be problematic for high temperature fuel cells that process hydrocarbon feed stocks. The presence of hydrocarbons at elevated temperatures also leads to the propensity to coke, which may limit such fuel cells to the use of hydrogen as the fuel source. However, a hydrogen fuel distribution network currently does not exist, and hydrogen storage has been a major materials and engineering challenge. Fuel cells which utilize a reforming step to produce H2 gas also require that the fuel cell power plant contains fuel reformers. Eliminating the reforming units would simplify the devices and may present significant benefits in costs savings and energy efficiency. The main obstacle to utilizing a hydrocarbon feed for SOFC power generation is the high nickel content in the traditional anode cermets such as Ni/YSZ. This material undergoes rapid coking which impairs performance and often causes mechanical damage to the electrolyte and/or fuel cell assembly.
Solid oxide fuel cells hold much promise for the direct conversion of hydrocarbon fuel to electricity. To allow for direct hydrocarbon feeds, the anode design may substitute nickel in the common cermet anodes with alternative electronic conductors that might be less prone to coking. Whereas efforts by others have accomplished this to some extent by replacement of nickel with other metals, it has required modification of standard synthetic techniques that may have deleterious effects on both durability and cost of the fuel cell. For example, some strategies have employed numerous impregnation cycles into a porous oxide host post electrolyte densification to metal volume percents typically around 15 volume percent.
Accordingly, improved materials and methods are needed.